Vacuum furnace with weight feed mechanism



United States Patent Helmut Scheidig Rossdorf;

Alfred M. Haul'f; Uwe Adolf Reimpell, Bruchkobel, Germany July 16,1969

Dec. 22, 1970 Leybold Heraeus Verwaltung G.m.b.H. Bonnerstrasse, Cologne-Bayental, Germany Sept. 10, 1968 Germany [72] Inventors [21 Appl. No. [22] Filed [45] Patented [73] Assignee Priority VACUUM FURNACE WITH WEIGHT FEED MECHANISM 4 Claims, 4 Drawing Figs.

US. Cl

51 1nt.Cl F27d 11/10 [50] Field ofSeai-ch 13/1,9,31, 12:73/(lnquired) [56] References Cited UNITED STATES PATENTS 3,179,734 4/1965 Redelet a1 13/12 3,272,905 9/1966 Wooding 13/9 Primary Examiner-Bernard A. Gilheany Assistant Examiner-Roy N. Envall, Jr. Attorney-Joseph F. Padlon ABSTRACT: A vacuum furnace for melting block-shaped charges, under seal having a feed mechanism longitudinally movable in the furnace and a mechanism for continuously weighing with high precision the melting block.

VACUUM FURNACE WITH WEIGHT FEED MECHANISM This invention relates to a vacuum furnace for melting block-shaped metalliccharges which is equipped with a feed mechanism for the charge passing through a seal in the furnace wall and longitudinally movable in the same, and provided with a device for continuously weighing the melting block with high precision. I

The value determined byweighing may be used for controlling electric heating current, controlling pressure and the like, or for controlling the feed movement. It is often preferred to control the feeding movement in accordance with the continuously determined weight of the charge because other parameters from which a control signal may be derived, such as the voltage and current at the electric arc of an electric arc furnace, provide extremely unreliable data relating to the spacing between the electrode and the melt which is to be controlled. Short circuits, splashing of the melt, gas eruptions, and voids or occluded bubbles in the charge may initiate control measures of the wrong magnitude or even of reversed direction. The continuous determination and evaluation of the charge weight is of great value in avoiding these shortcomings in indicating or controlling circuits. Thus, signals derived from the weight loss of the charge per unit time, that is, the differential quotient, when used for control purposes, make for a highly uniform melting process.

It is known from German Pat. No. 1,157,739 to arrange a force responsive resistor in a portion of the melting apparatus which is affected by the weight of the electrode,,and to transmit the electrical signals so obtained to a control device by way of an amplifier and a differentiating device, for controlling the melting rate. The known arrangement is substantially useful for furnaces which are equipped for the passage of the feeding mechanism through the furnace wall without frictional contact. If seals were used, sizable measuring errors as to the electrode weight would be causedby the unavoidable friction. Such errors or mistakes have particularly harmful effects in the determination of a difference because the variable frictional forces simulate a nonuniform melting rate and initiate control processes which may enhance this phenomenon. Moreover, the ,e'fiects of the frictional forces increase up to the end of the melting process because their effect increases relative to the decreasing electrode weight.

This shortcoming has been recognized, and it has been proposed in US. Pat. No. 3,272,905 to arrange the weight-indicating element at the junction of the feed mechanism to the electrode holder. In this manner, the frictional effects of a seal, if provided, are not measured, but'the weight indicating element is located within the furnace'where it is exposed to heat and passed by electric current which may amount to several tens of thousands of amperes. Moreover, such a weight indicating element, in'view of the great electrode weight and the leverage provided by the electrode cannot readily resist the hard or rigorous conditions of operation which necessarily causes a substantial reduction in the precision of measurement. It is absolutely necessary to provide intensive water cooling for the indicating device, because of the intense heat in the furnace.

In order to avoid the disadvantages of the known arrangements, it is proposed according to the invention fixedly to connect the seal for the feed mechanism in a vacuum furnace of the type described with a platform carrying the feed mechanism,.the platform being carried by at least one transducer for weight measurement, and the seal being connected with the furnace wall in a vacuum tight manner by means of an intermediate member which compensates for expansion.

The arrangement of the invention has the following advantages: It permits the transducer or transducers to be arranged without vacuum tight leads-outside the range of the thermal effects of the furnace and outside of the current path so that commercially available transducers may be employed. Frictional forces which may occur are compensated for, as will be described hereinafter, and cannot affect the measured values and the control process. The actual electrode weight which is uniquely determined, may thus be compared with a nominal weight, and the difference between the two values employed for a reliable, continuous control of the melting process. The term continuous weighing shall be understood as defining a weighing which is useful for recording or control, that is, to include individual weighings which are'performed at certain intervals. There is no danger of mechanical damage to the transducers because forces other than those to be measured have no effect or are safely intercepted.

As to friction, the feed mechanism, platform, and seal form a closed system of forces. The weights of the charge, of the feed mechanism with drive and platform and with accessory guides and rods, if any, is externally effective and transmitted to the transducers. The weights of all of these parts are known, and may be balanced in electrical circuits by means of potentiometers, or mechanically, as by counterweights.

It is not necessary to arrange the drive motor and the transmission for the feed mechanism on the platform, but the driving force may be transmitted to the platform from a motor arranged outside, for example through a universal joint and shaft or a telescoping shaft. However, a particularly simple and compact arrangement is obtained when the platform carries the drive mechanism or serves as the housing for the drive mechanism.

Furthermore, according to theinvention, a compensating member is associated with the aforementioned expansionbalancing intermediate member of the seal. ln response to the pressure difference between the atmosphere and the vacuum, the compensating member produces a force of such magnitude and direction that the force generated by the first mentioned expansion balancing intermediate member is nullified relative to the transducer for the determination of weight. Because of this arrangement, the measurement results are not continuously changed while the vacuum in the furnace chamber is established. It also becomes possible to operate at various fractional atmospheric pressures.

Ultimately, a well centered, vertical guidance for the feeding mechanism can be achieved without bulky supplemental structural elements by using the rigid connection between the seal and the platform as guide columns for the feeding mechanism.

The drawing herein illustrates several embodiments of the invention wherein:

FIGS. 1, 2 an 3 are sectional views of different variants of the invention; and

FIG. 4 is a section of a seal with an expansion balancing intermediate member.

Referring now to the drawings wherein like numerals refer to like parts throughout, there is shown in FIG. 1, a vacuum furnace 1 provided with an interior chamber la and a shell 2. Three transducers 3 for measuring-weight are supported on furnace shell 2. They are measuring cans with expanding measuring strips. Weight applied to the can causes a change in electrical resistance which provides a signal for the control process. A platform 4 rests on transducers 3 and functions like the bridge of a balance. A force-transmitting member consisting of two guide columns 5 and 6 extends upwardly from the platform. Two guide sleeves 7 and 8 are longitudinally slidable on said guide columns and are connected by a crossmember 9.

The upper ends of the guide columns are fixedly connected with a drive housing 10 which encloses a motor and transmission for the movement of the charge. These elements have not been illustrated for the sake of simplicity. The drive causes a threaded spindle ll cooperating with a nut 12 to rotate and raises or 'lowers the feed mechanism 13 when turning left or right, Because nut 12 is connected with crossmember 9, the sleeves 7 and 8 participate in the movement. Nut 12 constitutes the upper end of the feed mechanism 13 whose lower end is located in the furnace and is not shown. It is adapted to receive the charge. The feed mechanism passes the furnace wall at 14, a seal 15 being provided to maintain a vacuum in the furnace chamber 1a and permitting sliding movement of the feed mechanism 13.

Seal 15 is connected with a flange 17, sealed gas tight to the furnace wall, by means of an expansion-balancing intermediate member 16 which is a bellows of resilient material. The seal 15 moreover is connected fixedly and directly with the platform 4 whereby the platform receives the frictional forces generated by the seal, said 'forces occur according to the cross section of the intermediate member 16 in connection with the pressure difference relative to atmospheric pressure. In order to compensate for the last mentioned forces, a compensating member 18 in the form of bellows of resilient material of the same cross section is arranged on the opposite side of the platform 4. The compensating member 18 communicates with the interior of the furnace through a pressure equalizing line 19, and its reaction force is' absorbed by a C- shaped yoke 23. Said compensating member, however, may be replaced by suitable electric circuitry, such as potentiometers, etc. Platform 4 is prevented from shifting laterally by the use of two guide pins 411 mounted on the platform. The pins glide practically without friction in bearings 41. This structure, however, may be replaced with equal effect by'a known arrangement of horizontal links.

The apparatus shown in FIG. 1 is operated as follows:

When the feed mechanism 13 is moved in the direction of arrow 20, this movement, because of friction in the seal 15, produces a force in the direction of the arrow 21 which acts on platform 4. The force of reaction acts on the drive housing 10 in the direction of the arrow 22 by way of the threaded spindle 11. Since the drive housing and the platform are rigidly coupled by a force-transmitting element constituted by the guide columns 5 and 6, a closed system of forces is formed. There is no externally active resultant force which could act on transducers 3; The frictional forces between the guide sleeves 7 and 8 and the guide columns 5 and 6 are compensated in the same manner. Frictional forces are therefore excluded from the measured weight. Guide columns 5 and 6 transmit, in addition to frictional forces, the total pressure forces which ,are generated by the weight of the charge and of the drive. In FIG. 2 there is shown a modification of the apparatus of FIG. 1. The furnace shell 2 is extended upwardly by a closed frame 24 which envelops the drive housing 10 and the feed mechanism 13. The frame has two brackets 25 on which two transducers 3 are supported. The platform 4 and drive housing 10 constitute a continuous unit which is secured against lateral shifting by guide pins 40a and the bearing 41a. The feed mechanism 13 is driven and guided in a straight path as in FIG. 1. The seal 15 is fastened in a bridge 26 which transmits frictional forces to the guide columns 5 and 6 and from there to the platform 4, thereby closing the system of forces. Contrary to the arrangement of FIG. 1, guide columns 5 and 6 in FIG. 2 are relieved of the weight of the charge and of the drive. The seal 15 isagain connected with the furnace by an expansionbalancing intermediate member 16 and the compression forces generated by the intermediate member are compensated by a compensating member 18 acting on platform 4 in the opposite direction. Lateral shifting of the bridge 26 is prevented by guide pins 40 and bearing 41 or by an analogous linkage arrangement. It is to be noted that the apparatus of FIG. 2 has the advantage that transducers 3 are removed from the influence of the electric field in the furnace.

In FIG. 3 there is shown the basic arrangement of FIG. 2 adapted to a different typeof vacuum furnace. In this instance, the furnace is opened by lifting the upper part 2a of the furnace shell 2 in a vertical direction. Two hydraulic cylinders 27 engage the upper furnace part and the platform 4 or the drive housing 10 for this purpose, but are guided freely movably through the bridge 26 and the cross member'9. In order to make the upper furnace part movable, it is necessary that it be moved together with seal 15 and the attached bridge 26 relative to the guide columns over a relatively long distance. However, such a movement must not be effected In order not, unduly, to deform the expansion balancing intermediate member 16 during the lifting movement of the upper furnace part and the associated relative movement of the feed mechanism 13 and the seal 15, engaging claws 46 are fastened to the upper furnace, part and envelop the ends of the bridge 26 with some vertical clearance. The vertical clearance must be dimensioned so that the bridge 26, when locked, can freely follow the movements of the platform 4. A i

In order to prevent radial movement of the device, guide columns 5 and 6 are led through bridge 26, and their ends 30 are longitudinally movable in the upper furnace part. Because of the mobility of the upper furnace part, frame 24 is also made separate therefrom.

In FIG. 4 there is shown an example of a seal 15 which is particularly tight in combination with the expansion balancing intermediate member 16 which is a bellows. The bellows have a flat characteristic spring curve so that no measurable forces are transmitted-from the bellows to the platform 4 during the slight deformations consistent with the transducers 3. The seal proper comprises of a'housing 31 having annular oil seals 32 and 33 with sealinglips. Wiper rings 34 and 35 remove fine surface dirt originating in the furnace or in the atmosphere. A wiper ring 36 removes oil which fills the ring chamber 37 at a level controlled through a sight glass 38. The feed mechanism 13 is supported in a radial bearing 39. The bellows has upper I and lower fastening flanges 42 and 43. The upper flange simultaneously holds the holding tube 44 for the heat resistant protective bellows 45. The lower flange 46 is screwed to the flange 46 of the furnace shell 2. The seal 15 which can move V relative to the upper furnace part within certain limits is connected to by means of its housing 31 either directly with the platform 4 (FIG. 1) or with the bridge 26 (FIGS. 2 and 3).

FIG. 4 shows not only the arrangement of the individual parts in greater detail, but particularly'illustrates the wipers, seals, and bearing elements which cause the undesirable frictional forces.

We claim:

1. A vacuum furnace having a shell for the melting of blockshaped charges having a feed, mechanism which passes through the furnace wall and is longitudinally movable in the. same, and a mechanism for continuously weighing the melting block with high precision, comprising a platform, a seal fixedly connected with said platform carrying said feed mechanism, at least one transducer supporting said platform and used for weight determination, and being connected with said furnace shell, an expansion-balancing intermediate member in a vacuum tight manner, for said seal and said furnace shell.

2. A vacuum furnace according to claim 1 characterized in that the platform additionally serves for receiving the drive for the feed mechanism.

3. A vacuum furnace according to claim 1, characterized in that a compensating member is associated with the expansionbalancing intermediate member of the seal, the compensating member responding to the pressure difference between the atmosphere and the vacuum to produce a force of a magnitude and direction to nullify the force exerted by the first mentioned resilient intermediate member on the transducers.

4. A vacuum furnace according to claim 1, characterized in that the rigid connection between the seal and the platform is constituted by guide columns which guide the feed mechanism in a straight line. 

